1. Field of the Invention
The present invention relates to a process for the preparation of middle distillates starting from prevalently paraffinic charges.
More specifically, the present invention relates to a process for the production of middle distillates in a single reactive step comprising a hydrocracking reaction, starting from charges prevalently consisting of mixtures of n-paraffins in which a significant fraction has a boiling point higher than that of middle distillates.
2. Description of the Background
Mixtures of hydrocarbons comprising a relevant fraction with a high boiling point are normally obtained as distillation residue in refining processes of fuels of petroleum origin. More recently, mixtures of hydrocarbons with a high boiling point have also been obtained from certain degradation processes and recycling of polymeric materials. The production of hydrocarbon mixtures, essentially consisting of n-paraffins, in which a significant fraction has a boiling point exceeding 370xc2x0 C., by means of direct synthesis from mixtures of hydrogen and carbon monoxide (synthesis gas) in so-called Fischer-Tropsch processes, from the name of the inventors of the first synthesis of this type in the thirties"", is also well known.
The above mixtures are normally in solid or semi-solid form (waxes) at room temperature and in no case can be used as fuels or lubricants in this form. However, they represent a raw material which is potentially very significant as an energy source and for other uses, and are therefore subjected to various kinds of degradative and/or regradative treatment to improve their characteristics and allow them to be used as fuels. In particular, due also to the present tendency in the fields of automobile and air transport, in which there is an increasing demand for Jet Fuel and high quality gasolines for Diesel cycle engines, the necessity is strongly felt for obtaining, with the least possible number of steps and maximum yield, mixtures with typical properties of so-called xe2x80x9cmiddle distillatesxe2x80x9d.
The term middle distillates usually refers to a mixture of hydrocarbons with a range of boiling points corresponding to those of xe2x80x9ckerosenexe2x80x9d and xe2x80x9cgas oilxe2x80x9d fractions obtained during the atmospheric distillation of petroleum. In this distillation, the boiling point range which defines the xe2x80x9cmiddle distillatexe2x80x9d generally ranges from 150 to 370xc2x0 C. The middle distillate cut consists in turn of: 1) one or more kerosene fractions with a boiling range generally between 150 and 250xc2x0 C.; 2) one or more gas oil fractions with a boiling range generally between 180 and 370xc2x0 C.
It is known that hydrocarbon mixtures suitable for producing the above fuel cuts, with a good yield, after fractionation, can be obtained by subjecting a high-boiling mixture of hydrocarbons, normally having a distillation range exceeding 350xc2x0 C., to a degradative thermal process in the presence of hydrogen. These processes, more commonly defined as xe2x80x9chydrocrackingxe2x80x9d, are normally carried out in the presence of a bifunctional catalyst, containing a metal with a hydrogenating activity supported on an inorganic solid usually comprising an oxide or a mixture of oxides with acid characteristics.
Hydrocracking catalysts typically comprise metals of groups 6 to 10 of the periodic table of elements (in the form approved by IUPAC and published by xe2x80x9cCRC Press Inc.xe2x80x9d in 1989, to which reference is continually made hereafter), especially nickel, cobalt, molybdenum , tungsten or noble metals such as palladium or platinum. Whereas the former are more suitable for processing hydrocarbon mixtures with relatively high sulfur contents, the noble metals are more active but are poisoned by the sulfur and require a feeding essentially without its presence.
Carriers normally used for the purpose are various types of zeolites (xcex2, Y), X-Al2O3 (wherein X can be Cl or F), silico-aluminas, the latter amorphous or with varying degrees of crystallinity or mixtures of crystalline zeolites and amorphous oxides. A very detailed description of the various catalysts, specific characteristics and different hydrocracking processes based on these, is provided, among the many available in literature, in the publication of J. Scherzer and A. J. Gruia xe2x80x9cHydrocracking Science and Technologyxe2x80x9d, Marcel Dekker, Inc. Publisher (1996).
The availability of high-boiling mixtures or waxes, produced directly, for example, by means of synthesis processes of the Fischer-Tropsch type, although greatly desired (absence of polycondensed aromatic compounds, asphaltenes, sulfur and nitrogen), requires however a particular selection of catalysts and process conditions which makes this alternative possible at costs competitive with the traditional sources of liquid mineral fuels.
In fact, owing to the particular growth mechanism of the hydrocarbon chains during the Fischer-Tropsch reaction, it is not achievable in practice the synthesis of a product with a narrow chain-length range. Regardless of the type of catalyst and operating conditions, the Fischer-Tropsch reaction produces a mixture of products characterized by an extremely wide molecular weight distribution, ranging from methane to normal-paraffin waxes, even containing more than 100 carbon atoms. An appropriate choice of synthesis catalyst and operating conditions allows the type of product to be varied in terms of the relative content of paraffin, olefin and oxygenated compounds in the mixture, and average hydrocarbon chain length. It is consequently possible to obtain different mixtures of hydrocarbons with a composition more or less approaching heavy products, but always with a relatively wide distribution. For example, in the case of the products obtained with catalytic systems of the most recent generation based on cobalt, which tend to produce long-chain hydrocarbons, generally only 40-60% of the hydrocarbon fraction having at least 7 carbon atoms (abbreviation C7+) consists of a middle distillate, whereas the complement to 100% consists of heavier products.
A critical element in the hydrocracking process of products coming from the Fischer-Tropsch synthesis is the reactivity of the molecules with an increase in the paraffinic chain length. To obtain high selectivities to middle distillates, using a xe2x80x9cfull rangexe2x80x9d charge (i.e. C5+), it is necessary for the reactivity towards cracking the components of the naphtha, kerosene and gas oil fractions to be much lower than that of the fraction having a boiling point higher than 370xc2x0 C. (370+xc2x0 C. fraction). Otherwise, during the hydrocracking reaction, a fraction consisting of middle distillates is converted to gas (C1-C4) and naphtha (C5-C9) with a consequent decrease in the selectivity to middle distillates. Another consequence which derives from not respecting the above condition is the considerable increase in the kerosene/gas oil ratio, during the reaction, even at relatively low conversion degrees of the heavy fraction (370+xc2x0 C.). This is a particularly negative aspect if the yields to gas oil are to be maximized.
In the processes of the known art, there is generally a considerable increase in the kerosene/gas oil ratio and a consistent decrease in the selectivity to middle distillates at high conversion degrees of the 370+xc2x0 C. fraction. In order to limit these drawbacks, when C5+ or 150+xc2x0 C. charges are processed, it is normal to operate with conversions of the heavy fraction (360+xc2x0 C.) ranging from 35 to 60%. Alternatively, the hydrocracking reaction is only carried out on 260+xc2x0 C. or 370+xc2x0 C. cuts obtained by means of a preliminary fractionation step.
Another critical aspect of hydrocracking processes of Fischer-Tropsch synthesis products is the isomerization degree of the products which, with the same range of boiling points, strongly influences their properties at low temperatures. In this respect, to obtain products which satisfy the specifications for use as fuels for car transport (Diesel) and air transport (Jet Fuel), the percentage of iso-paraffins must be relatively high. The production of middle distillates containing high percentages of iso-paraffins together with the achievement of high selectivities, however, is still a critical aspect which is faced by subjecting whole or part of the middle distillates to an isomerization process with the purpose of improving the properties at low temperatures. In any case, the end process is more complex and greater investment costs are necessary.
Various catalytic systems have been proposed in the art in an attempt to overcome the above problems. For example, in the publications of patent applications GB-A 2,077,289, EP-A 104,672, EP-A 109,702, EP-A 147,873 and EP 127,220, various mixtures of hydrocarbons coming from the Fischer-Tropsch synthesis are treated with hydrogen in the presence of a catalyst consisting of platinum supported on amorphous silica-alumina having a high alumina content ( greater than 10% by weight). In the examples provided, there is an increase in the content of middle distillates but, when indicated, the pour point of the gas oil fractions and the freezing point of the kerosene fraction are xe2x88x921xc2x0 C. and xe2x88x9235xc2x0 C. respectively.
Patent applications EP-A 532,115, EP-A 532,116 and EP-A 532,117 also describe the use of catalysts based on platinum supported on amorphous silica-alumina for the production of middle distillates starting from high-boiling mixtures coming from the Fischer-Tropsch synthesis. In particular the use of amorphous silico-aluminas containing from 12 to 15% by weight of alumina with a pore volume, determined by means of xe2x80x9cincipient wetnessxe2x80x9d, preferably ranging from 1 to 1.5 ml/g, is described. The examples provided, relating to the conversion of a 370+xc2x0 C. fraction, indicate selectivities to middle distillates (220-370xc2x0 C. fraction) ranging from 0.50 to 0.59 within the conversion range of 44 to 86%. No information is provided as to the treatment of high-boiling mixtures with a significant initial content of middle distillates, nor to the characteristics at low temperatures of the products obtained.
Patent application EP-A 321,303 discloses a process which comprises the separation of the light fraction (290xe2x88x92xc2x0 C.) of Fischer-Tropsch products, and sending the 290+xc2x0 C. fraction to a hydrocracking/isomerization reactor for the production of middle distillates. The non-converted 370+xc2x0 C. fraction can be recycled to the hydrocracking reactor or optionally sent, either entirely or partly, to a second isomerization reactor, for a further production of Jet Fuel and lube bases. The catalyst claimed for both reactors consists of platinum supported on fluorinated alumina. The examples provided indicate that by feeding the hydrocracking reactor with a 370+xc2x0 C. charge, maximum yields of about 50% are obtained for a conversion of the charge ranging from 70 to 90%.
U.S. Pat. No. 5,378,348 describes a process in numerous steps for the treatment of paraffinic waxes which comprises the separation of the charge into three fractions: 1) naphtha (C5xe2x88x92165xc2x0 C.); 2) kerosene (160-260xc2x0 C.); 3) residue (260+xc2x0 C.). The kerosene fraction is subjected to a process in two steps: the first, a hydrotreating process to remove the olefins and oxygenated compounds; the second a hydroisomerization process to improve the properties at low temperatures. The 260+xc2x0 C. fraction is sent to a hydrocracking/isomerization reactor for the production of middle distillates, and the non-converted 370+xc2x0 C. fraction is recycled. The advantages deriving from the use of this scheme are higher yields to middle distillates and good properties at low temperatures. Preferred catalysts are based on a noble metal (Pt, Pd) or the pairs Ni+Co/Mo on silica alumina or silica-alumina modified by impregnation of the carrier with a precursor of silica (e.g. Si(OC2H5)4). The examples relating to the conversion of the 260+xc2x0 C. fraction, using different catalysts, indicate kerosene/gas oil ratios ranging from 0.63 to 1.1 for a conversion of 39-53% of the 370+xc2x0 C. fraction. The freezing points of the 160-260xc2x0 C. cut range from xe2x88x9243 to xe2x88x9225xc2x0 C. whereas the pour points of the 260-370xc2x0 C. fraction vary from xe2x88x923 to xe2x88x9227xc2x0 C.
From an examination of literature available, it seems that there has never been a description of a process, operating directly on a C7+ fraction, capable of producing in a single step and with high conversion degrees of the 370+xc2x0 C. fraction (conversion  greater than 60%), middle distillates with high selectivities and a minimum variation in the kerosene/gas oil ratio with respect to the charged processed. The necessity has also been felt for having a catalytic system which gives rise, starting from waxy hydrocarbons and particularly from mixtures of high-boiling hydrocarbons obtained from Fischer-Tropsch processes, to the formation of middle distillates with good properties at low temperatures in terms of freezing point of the kerosene fraction and pour point of the gas oil fraction.
These characteristics, in fact, are associated with the branching degree and are difficult to optimize in a process comprising hydrocracking as single reactive step, as satisfactory branchings can usually be obtained with conversion rates which are too high, at which the gas oil fraction is excessively degraded to lighter fractions. The result is that a high branching degree seems to be incompatible with the production of a balanced conversion to kerosene and gas oil.
It is therefore necessary to have catalytic systems for hydrocracking which are further improved with respect to those used in the processes so far known, and which overcome the above drawbacks, especially in the case of the treatment of prevalently linear aliphatic hydrocarbon mixtures, such as those produced in Fischer-Tropsch processes.
Patent application EP-A 582,347 (assigned to the Applicant) describes a catalytic system capable of isomerizing with high selectivities n-paraffins having a number of carbon atoms exceeding 15 which comprises:
1. a carrier of an acid nature consisting of a silica or alumina gel amorphous to X-rays, with a molar ratio ranging from 30/1 to 500/1, having a surface area ranging from 500 to 1000 m2/g, a porosity ranging from 0.3 to 0.6 ml/g and a pore diameter within the range of 10-40 xc3x85
2. one or more noble metals of groups 8, 9 or 10 of the periodic table, deposited in a quantity ranging from 0.05 to 5% by weight.
The acid carrier of the catalyst preferably has a ratio SiO2/Al2O3 ranging from 50/1 to 300/1 and a porosity ranging from 0.4 to 0.5 ml/g, whereas the metal or mixture of noble metals consists of platinum and/or palladium in a quantity ranging from 0.1 to 2.0% by weight.
The carrier based on silica and alumina gel can be conveniently prepared according to what is described in patent application EP-A 659,478. Said carrier can be used as such for the deposition of the metal phase (b) or in extruded form, as described for example in EP-A 550,922 and EP-A 66055. The metal phase (b) of the catalyst can be introduced by means of aqueous impregnation by wetting the extruded carrier with an aqueous solution of a compound of the desired metal or metals, or according to the method described in patent application EP-A 590,714. This is followed by calcination in an oxidating atmosphere at a temperature ranging from 200 to 600xc2x0 C. These catalytic compositions can be used in the hydroisomerization of n-paraffins having over 15 carbon atoms to obtain bases for lubricating oils characterized by a low pour point and a high viscosity index.
During a varied series of tests on the behaviour of the above isomerization catalysts, the Applicant surprisingly found a particular type of catalytic composition which proved to be extremely advantageous when used in hydrocracking processes, and at the same time relatively simple to prepare, thus allowing evident progress in overcoming the drawbacks described above.
An object of the present invention therefore relates to a process for the preparation of middle distillates starting from a mixture of substantially linear hydrocarbons, consisting of at least 20% of a high-boiling fraction having a distillation temperature exceeding 370xc2x0 C., said process comprising a hydrocracking step in which said mixture of linear hydrocarbons is heated in the presence of hydrogen to a temperature ranging from 250 to 450xc2x0 C. and at a pressure ranging from 0.5 to 15 MPa, for a time sufficient for converting at least 40%, preferably from 60 to 95%, of said high-boiling fraction into a fraction of hydrocarbons which can be distilled at a temperature lower than 370xc2x0 C.; this process is characterized in that: the hydrocracking step is carried out in the presence of a supported catalyst comprising:
(A) a carrier of an acid nature consisting of a calcined gel of silica or alumina amorphous to X-rays, having a molar ratio SiO2/Al2O3 between silica and alumina within the range of 30 and 500, a surface area ranging from 500 to 1000 m2/g, with a porosity ranging from 0.2 to 0.8 ml/g, preferably from 0.3 to 0.6 ml/g, with an average pore diameter within the range of 10 and 40xc2x710xe2x88x9210 m (between 10 and 40 xc3x85);
(B) one or more noble metals of groups 8, 9 or 10 of the periodic table, deposited on said carrier (A) in a quantity ranging from 0.05 to 5% by weight.